The dielectric breakdown voltage of third-generation semiconductor Gallium Nitride (GaN) is as high as 3MV/cm and is much higher than that of first-generation semiconductor silicon (Si) or second-generation semiconductor gallium arsenide (GaAs), and thus an electronic device of GaN can withstand a very high voltage. The channel of a GaN hetero-junction structure has a very high electron concentration and very high electron mobility. This means that a Gallium Nitride high electron mobility transistor can conduct high current at high frequency and has a very low turn-on resistance. In addition, GaN is a wide bandgap semiconductor and may operate at high temperature. Those characteristics as described above make GaN HEMT especially suitable as a high frequency and high power radio frequency device, or as a high voltage switching device.
A gallium nitride HEMT is generally a depletion-mode field effect transistor or referred to as a normally-on device, since the AlGaN/GaN hetero-junction channel formed by piezoelectric and spontaneous polarizations has a very high concentration of two-dimensional electron gas (2DEG). There also exist normally-off devices, which also referred as enhancement-mode devices, in contrary to normally-on devices. The application of depletion-mode devices has limitations. In radio frequency power amplification area, a depletion-mode device must employ a negative bias voltage to the gate electrode, which requires the system to provide a completely independent power supply. In electrical power conversion filed, the application of a depletion-mode switching device not only needs an independent negative bias circuit as described above, but also requires the independent negative bias circuit to be powered up before the whole power conversation system is powered up, which is normally hard to realize. An enhancement-mode device is necessary in a power conversion system to avoid device failure by sudden rising of conducting current during system powering up.
Currently, normal methods to achieve an enhancement-mode gallium nitride HEMT include recessed gate structure, fluorine plasma bombardment treatment to the gate metal contact region, and so on. FIG. 1 shows a GaN HEMT having the recessed gate structure. A substrate 12 for growing the GaN material normally is sapphire, SiC or silicon. A nucleation layer 13 is grown on the substrate 12; a GaN epitaxial layer 101 is grown on the nucleation layer 13; an AlGaN layer 102 is grown on the GaN epitaxial layer 101. In this case, the two-dimensional electron gas (2DEG) may occur at the interface between the AlGaN and GaN layers and thus forms a channel. Two ohmic contacts form a source 22 and a drain 23 of the field effect transistor, respectively. In the region between the source 22 and the drain 23, the AlGaN is etched to form a trench, and then a metal gate 104 is formed in the trench. The trench and the metal gate formed therein is referred to as recessed gate structure. In the case that the AlGaN layer is thin enough, the 2DEG will be depleted, and therefore there is no electron in the channel under the gate. Such structure is referred to as an enhancement-mode field effect transistor since the channel thereof is pinched off under zero gate bias voltage. However, because of a very strong polarized electric field existing in the AlGaN layer, there may occur electrons in the channel even though the thickness of the AlGaN layer is very thin. As a result, for an enhancement-mode device with recessed gate structure, the thickness of the AlGaN layer under the metal gate must be thinned to a range of 3 nm to 5 nm, or below, by dry etching. It is very difficult to control an etching process with such a high accuracy. Thus, the pinch-off voltage of the device shows a large fluctuation. Moreover, the pinch-off effectiveness of such structure is limited because of a low pinch-off voltage, so there remains a small amount of channel leakage current even at zero bias. When the device operates at a high voltage, the channel leakage current can easily cause the burnout of the device. Therefore, such device structure is not practical.
FIG. 2 shows an enhancement-mode GaN HEMT with fluorine plasma bombardment treatment to the gate metal contact region. The processes prior to forming the source 22 and the drain 23 are the same as those of a recessed gate GaN HEMT. After forming the source and drain, the region under the gate is subjected to fluorine plasma bombardment prior to depositing the metal gate 114. The crystal structure of the AlGaN layer 115 subjected to the fluorine plasma bombardment is damaged, leading to depletion of 2DEG in the channel 118 under the AlGaN layer 115, and thus forming an enhancement-mode field effect transistor. The reliability of such device has not been verified since the crystal structure is damaged. Furthermore, the fluorine atom is small. In the case that the device operates at high temperature and high voltage conditions for a long period, the fluorine atom may be released from AlGaN. It is possible that the enhancement-mode transistor turns back to a depletion-mode transistor, causing the system using such device to fail and be damaged.